Monday, October 28, 2013

Jazz Disassemblies Ep3: Sega Genesis Saving Teardown

I love the Sega Genesis, but rarely do anything with it. I have made a handful of reproduction carts in the past because there is nothing better than playing games on real hardware but I always have a regret after destroying a donor cartridge, whether it is sports or not. One of the goals I always set myself up with during a creating is to use 100% renewable components. By renewable, I mean of course modern and commercially produced components; nothing salvaged and nothing obsolete. Having nothing obsolete is very difficult in my circle of interests, but salvaging components is my greatest downfall.

During my journeys into the technologies that Sega and other companies used within the cartridges for the Sega Genesis/MegaDrive, I have found three official forms of saving data. Two forms use serial EEPROMs and the third uses the tried-and-true parallel SRAM with battery backup. My goal is to recreate these cartridges using new components and my stretch goal is to expand the addressable memory or just to improve them in some way while making the carts themselves renewable.

We already know that the plastic shells can be remade by the everyday hobbyist and their 3D printers and circuit boards can be fabricated by Chinese companies for pennies on the dollar. All that leave now is the components which are definitely on their way out of style. The components that we are looking for are 8- and 16-bit, parallel ROMs and RAMs. Referring to a statement above: by renewable I meant purchasable from Mouser or Digikey, etc in large quantities which will later be restored. Glancing at Mouser (My supplier of choice) I have already been able to locate a handful of ROMs and RAMs that come in both selectable 8- and 16-bit configurations!

The next goal I will have is to recreate the higher density logic with PLDs, but that is best left for another time.

1. KM62256 Parallel SRAM
These boards found in most of the miserable football and soccer games. Until I find another varient, I will cover one such board revision: "171-6279A"
The board seems to be made by Sega though I have long lost the ROM which was originally soldered in. It contains:

I have already recreated everything on the board in Eagle PCB libraries including the board dimensions and general component layout. Tracing all of the connections is slower work and I will get to that eventually. In the meantime, my goal is to layout a functionally-identical board with 3v3 ROM/RAM, SMT caps and level shifters for proper data flow. THen we will have ourselves a flash cartridge!

Above you have seen the board itself with and without components. Ignore the text on the Mask ROM though since I just stuck a random IC in there to show it with one.

Looks pretty good if I do say so myself, though the traces are not as authentic as the layout. Sega never seemed to use top-side pads. They have vias which allow for double sided boards, but I have only seen EA cartridges that use top-side pads. These pads of course make for miserable desoldering since I need much more heat... and patience.

Some fun facts about this board is that the ROM is 16-bits but the SRAM is only 8-bit. Although I do not have the full details on how the software accesses these, the Gen/MD has two pins which are called !LDSW and !UDSW (Upper Data Set Write and Lower Data Set Write). When reading from RAM, the processor ignores the upper byte of data since there should be nothing there. While writing though, the !LDSW pin goes low which enables the !WE pin on our RAM. These two pins are for transferring 8-bits (one byte) at a time rather than 16 (two bytes).Not knowing how to activate either of these pins, it would seem that someone may add a second SRAM and use the currently unused !UDSW pin as the enable.

The above picture is how I found the glue logic for addressing our memories. Only 6 out of 8 gates are used which is a waste of space and battery power since the unused pins are connected to the Vout pin on our reset IC. If you think about it, the three NAND gates that have both inputs connected act as NOT gates which is something we would take into consideration if we were to redesign this with single-gate SMT ICs or on a PLD.

one problem I have with this board is that A21 and A22 are simply left hanging. Just a guess, but using A20 as a ROm address and A21 in the logic would expand the addressable memory, but Sega chose not to for some reason.

2. Acclaim Serial 24LC02B RAM
The next board I will cover is the P/N 670120 REV 2 by Acclaim. The contents of our board are as follow:

As you can see, the naming routines is different than that of Sega and yet again, we could reduce the chip count to much less with a PLD. U2-U4 could easily be designed in a PLD to reduce space and cost. For the time being, I assume the resistors are pull-ups or pull-downs.

3. Acclaim Serial 24LC04B RAM w/ LZ95A53

You're probably thinking that I recycled this board from a previous post and yes, yes I have. It does pertain to the topic though and I can probably shed a little more light on the special IC now that I know more about !LDSW and serial eeproms.

Above is the board that I created by probing all of the traces. Looks nice, but my next goal would be to reverse engineer the Acclaim's LZ95A53. Unfortunately, I have no scope to do so...

The above picture is my schematic which shows the connections on the LZ95A53. I had to make an addition to my cartridge connector since it uses several different pins that very few others use. I believe that the Acclaim's LZ95A53 IC contains the same logic as the board which used the 24lc02 serial RAM. Again, I cannot test this theory.

In hindsight, all of these boards used 27c160 equivalent Mask ROMs. The 27c160 can store a 2MB ROM which means the board with 2 Mask ROMs had a 4MB game. A piece of information for those making reproduction carts with the 27c400, 800, 160 and 322's, the first three mentioned all have a !BYTE pin. This pin allows for the EPROM to function as either an 8-bit or 16-bit ROM which means you may use it in many different systems.

Besides a little bit of work on the silkscreens, these boards are all ready to send to any fab house, granted they make 1.6mm thick boards. I will also be adding some more pictures of the other two boards shortly. Thanks for reading.

I'll see if I can gather my schematics and libraries. There are a lot of libraries that I had to create from scratch. In fact, I go out of my way to design all of the default libraries by myself so they fit my ideal rules. I am very picky. :P

As for a CPLD, I don't think that would be necessary unless you were going to map a massive amount of ROM/RAM that never came on an official cart. I have never seen a board using sram (or fram) that required more than ONE logic chip. A CPLD would just be too expensive and complex for that purpose. Even a board using Serial RAM only needed a max of three logic chips. But in this case, half of each chip seemed to be unused so a cpld might be useful. Did you read my post on the accliam chip?

Hey, thanks for your work! I just found your blog and it's amazing how much reverse engineering you've done. I also would like to have the Acclaim boards in Eagle files, I have plans to build my own carts for the genesis. Thanks!! Please send to dreamingdies@gmail.com